The inventions described below were developed with the goal of providing new and better therapies for certain types of vascular disease for which the present day therapies are widely regarded as inadequate. Vascular disease includes aneurysms which can rupture and cause hemorrhage, atherosclerosis which can cause the occlusion of the blood vessels, vascular malformation and tumors. Occlusion of the coronary arteries, for example, is a common cause of heart attack. Vessel occlusion or rupture of an aneurysm within the brain are causes of stroke. Tumors fed by intra-cranial arteries can grow within the brain to the point where they cause a mass effect. The mass and size of the tumor can cause a stroke or the symptoms of stroke, requiring surgery for removal of the tumor or other remedial intervention.
Other therapies for occlusions of various arteries are under development. Balloon angioplasty is a technique in which a balloon is inserted into a stenosis which occludes or partially occludes an artery and is inflated in order to open the artery. Atherectomy is a technique in which occlusive atheromas are cut from the inner surface of the arteries. The newly preferred therapy for coronary occlusions is placement of an expanded metal wire-frame, called a stent, within the occluded region of the blood vessel to hold it open. Stents of various construction have been proposed, including the Palmaz-Schatz™ balloon expandable metal stent, the Wallstent self-expanding braided metal stent, the Strecker knitted metal stent, the Instent™ coil stent, the Cragg coiled stent and the Gianturco Z stent. Stents have been proposed for treatment of atherosclerosis in the neck, but carotid endarterectomy is still the preferred treatment for stenosis. Most perioperative strokes are thought to be caused by technical errors during endarterectomy (see Becker, Should Metallic Vascular Stents Be Used To Treat Cerebrovascular Occlusive Disease, 191 Radiology 309 (1994)). The same concerns militate against other forms of therapy such as angioplasty for treatment of the carotid arteries. Various factors, including poor long-term patency, distal emboli causing a stroke, the potential for crushing from external pressure, and the need for long term anti-coagulation, lead to the avoidance of certain stents in vessels smaller than the iliac arteries or in locations susceptible to external pressure. See, for example, Hull, The Wallstent in Peripheral Vascular Disease, For Iliac Use Only, 6 JVIR 884 (November-December 1995).
Stent-grafts have been proposed and used to treat aneurysms in the large blood vessels such as the aorta, and these typically include tube graft material supported by a metallic stent. These stent-grafts are designed for use in the large blood vessels, and the various layers of stents and grafts make them unsuitable for use in smaller blood vessels. Stent-grafts are not currently used in the coronary arteries which are typically 3 or 4 mm in internal diameter. Rolled stents have been proposed for use in aortic aneurysms. For example, Lane, Self Expanding Vascular Endoprosthesis for Aneurysms, U.S. Pat. No. 5,405,379 (Apr. 11, 1995) suggests the use of a polypropylene sheet placed in the abdominal or thoracic aorta to bridge aneurysms. It is particularly emphasized in Lane that the rolled sheet must be imperforate. Winston, Stent Construction of Rolled Configuration, U.S. Pat. No. 5,306,294 (Apr. 26, 1994) proposes a rolled sheet of stainless steel. Neither device has been approved for use in humans. The Winston device has not been used in humans. Of similar construction are the single layer rolled stents such as Kreamer, Intraluminal Graft, U.S. Pat. No. 4,740,207 (Apr. 26, 1988) and its reissue U.S. Pat. No. Re 34,327 (Jul. 27, 1993), which are expanded by balloon and include a ratchet mechanism which projects into the lumen of the stent, thus making it unsuitable for critical vessels in the brain and small diameter vessels. Khosravi, Ratcheting Stent, U.S. Pat. No. 5,441,155 (Aug. 15, 1995) and Sigwart, Intravascular Stent, U.S. Pat. No. 5,443,500 (Aug. 22, 1995) are other examples of rolled stents with ratcheting locking mechanisms.
Aneurysms of peripheral arteries and arteries of the neck have been treated experimentally with open walled stents such as the Strecker braided stent. Szikora, et al., Combined use of Stents and Coils to treat Experimental Wide-Necked Carotid Aneurysms, 15 AJNR 1091 (June 1994) illustrates use of a Strecker stent in the proximal vertebral arteries in dogs, and teaches that an open walled or porous stent is required to avoid excessive ingrowth. The Strecker stent has a small metal to blood vessel surface ratio, and has large openings between each of the wires making up the stent. The current technique in the use of open walled stents in the aneurysms of peripheral arteries is based on the theory that placement of the open walled stent slows the blood flow in the aneurismal sac, leading eventually to the formation of clots and fibrous masses which occlude the aneurysm. This technique has been combined with placement of micro-coils through the wall of the stent and into the aneurysm to further encourage fibrous tissue development within the aneurysm. The Szikora article and others show that knitted stents have not been effective in isolating an aneurysm from the circulatory system. Another problem noted with this technique is that blood clots can escape the open walled stent.
Stents have not previously been used for aneurysms of the blood vessels in the brain. The vessels in the brain likely to develop stenosis, aneurysms, AVM's and side branches requiring occlusion have diameters of about 1 mm to 5 mm, and can be accessed only via highly tortuous routes through the vascular system. Instead, surgical clipping, resection, complete occlusion with acrylic-based adhesives (super glue) or small balloons (thereby intentionally occluding the downstream portion of the blood vessel and any portion of the brain supplied by that portion), stuffing with foreign objects, etc. have been used. In a method of current interest, small coils are stuffed into the aneurysm via a catheter. One such small coil is known as the Guglielmi Detachable Coil or GDC. After placement of a few coils, which partially obstruct blood flow in the aneurysm, the blood clots or fibrous matter forms within the sac. This technique has reportedly resulted in clots and coils falling out of the sac, and the technique is not used on wide-neck aneurysms. Aneurysm clipping, in which the skull is opened and the brain dissected to expose the outside of the aneurysm, followed by placement of clips at the base of the aneurysm, is also an option for treatment. However, these techniques do not always effect an immediate and complete seal of the aneurysm from the high pressure of the circulatory system, and rupture, leakage and deadly complications occur. Aneurysm rupture and bleeding during surgical clipping and shortly after the clip placement is a significant problem and add difficulty to the procedure. Examples of the problems inherent in the use of both GDC's and aneurysm clips are illustrated in Civit, et al., Aneurysm Clipping After Endovascular Treatment With Coils, 38 Neurosurgery 955 (May 1996) which reports that several patients in the study died after unsuccessful coil placement and before they could be re-treated with the open skull clip placement. Thus the article illustrates that GDC's do not always work, and when they fail they may leave the patient in a critical condition. As illustrated in the article, bleeding during surgical clipping and shortly after the clip placement is also a frequent problem.
From experiences like this, it is apparent that the ultimate goal of intracranial aneurysm treatment is the complete or nearly complete exclusion of the aneurysm cavity from the circulation, which prevents bleeding into the brain cavity and prevents formation of distal blood clots. This goal may be achieved immediately to ensure successful treatment by means of a substantially imperforate stent. It may also be achieved with a slightly perforated stent which alters flow in such a way that compete clotting, over time, is initiated within the aneurysm. It may also be achieved with a perforate stent through which embolic material such as coils are placed in the aneurysm. The treatments may be accomplished by placement of the stents described below which generally do not require the use of balloons for expansion of the stent, so that the blood vessel being treated is not occluded during placement of the stent.
Typically, the stents described below will be delivered percutaneously, introduced into the body through the femoral artery, steered upwardly through the aorta, vena cava, carotid or vertebral artery, and into the various blood vessels of the brain. Further insertion into the brain requires passage through the highly tortuous and small diameter intra-cranial blood vessels. The Circle of Willis, a network of blood vessels which is central to the intracranial vascular system, is characterized by numerous small arteries and bends. Passage of a stent from the internal carotid through the Circle of Willis and into the anterior cerebral artery (for example) requires a turn of about 60° through blood vessels of only 1-5 mm in diameter. Clinically, many significant aneurysms take place in the Circle of Willis and approaching blood vessels. The stent and delivery systems described herein are intended for use in such highly tortuous vessels, particularly in the Circle of Willis, the vertebral and carotid siphons and other major blood vessels of the brain. At times, pathologically tortuous vessels may be encountered in the deeper vessels of the brain, and these vessels may be characterized by small diameter, by branching at angles in excess of 90° and by inaccessibility with guide wires larger than the standard 0.018 guide-wires. These pathologically tortuous vessels may also be subject to aneurysms and AVM's which can be treated with the stents and delivery systems described below.
Various inventors have proposed systems for delivering stents into the larger less tortuous vasculature of the abdomen, chest, arms and legs. Garza, Prosthesis System and Method, U.S. Pat. No. 4,665,918 (1987), describes a delivery system for a stent designed to open an occlusion in a blood vessel. The stent is rolled on a catheter, and the catheter is covered with a sheath that fits closely over the catheter. The stent is held in place on the catheter by the sheath, and is released by pulling the sheath backward (proximally) to uncover the stent.
Winston, Stent Construction of Rolled Configuration, U.S. Pat. No. 5,306,294 (Apr. 26, 1994) shows a rolled sheet stent intended for use in bridging aneurysms and opening occluded blood vessels. The stent is self expanding, and does not require a balloon to force it to unwind to a diameter large enough to engage the aorta with enough force to hold it in place. The delivery system shown in Winston includes a rolled stent rolled upon a spool which in turn is mounted on the distal tip of a delivery catheter. The assembly is housed within the lumen in the distal tip of a guide catheter. Control wires shown in Winston may be used to hold the stent in the tightly wound state, and may be pulled proximally to release the stent.
Lane, Self Expanding Vascular Endoprosthesis for Aneurysms, U.S. Pat. No. 5,405,379 (Apr. 11, 1995) shows a rolled sheet stent intended for use in bridging abdominal aneurysms. The stent is self expanding, and does not require a balloon to force it to unwind to a diameter large enough to engage the aorta with enough force to hold it in place. The delivery system shown in Lane includes a rolled stent inside the lumen in the distal tip of a delivery catheter, and a push rod located behind the stent. The stent is deployed when the push rod is used to push the stent distally out the end of the delivery catheter as the delivery catheter is pulled proximally to uncover the stent and allow it to unwind.
Kreamer, Intraluminal Graft, U.S. Pat. No. 4,740,207 (Apr. 26, 1988) shows a tube stent, comprising a solid walled tube with ratcheting tongue and groove mechanism. The stent is mounted on an angioplasty balloon and expanded to size within an artery by force of the balloon. Likewise, Sigwart, Intravascular Stent, U.S. Pat. No. 5,443,500 (Aug. 22, 1995) shows a rolled stent, comprising a rolled lattice with ratcheting mechanism. The stent mounted on an angioplasty balloon and expanded into place by force of the balloon. A pull wire threaded through the lattice of the rolled stent helps hold the stent in place on the balloon during delivery through the vasculature. Sigwart does not discuss the mechanisms and methods for delivering the stent to the target site within the blood vessel.